1. Field of the Invention
The present invention pertains to the classification of particulate solids in chemical manufacturing processes, more particularly, to the efficient removal of large particulates, agglomerates, and foreign matter from a gas-transported stream of particulate product.
2. Background Art
In chemical manufacturing processes, it is frequently desired to provide a pulverulent or nearly dry product of defined particle size range. For example, in many chemical processes, a moist product filter cake is obtained which is broken up and dried in a gas stream, for example in a fluidized bed dryer. A product stream containing product particles depleted of water and/or organic solvents is conveyed by entertainment in a gas stream, to a packaging or shipping station. Additional “drying” may take place in the conveying gas stream, and the product may be completely dry or may still contain traces of liquid.
For many products, a defined range of particle sizes is desired, and freedom from large particulates and agglomerates is often a necessary requirement. Large particulates may be artifacts of crystallization processes employed to isolate and/or purify the product. Agglomerates may be created during these processes as well, or during drying in the drying apparatus. Creation of agglomerates or “sintering” is more likely to occur with products which are inherently tacky, or where the drying temperature is close to the product softening or melting temperature. Low gas velocities in fluid bed dryers generally exacerbate such large particle formation. Large particles may also result from sloughing off of product accumulated on reactor walls or in the dryer or conveying lines. Such particles may or may not have the same chemical composition as the desired product. Foreign matter such as metal pieces, deteriorated pump seals, etc., may be introduced into the product at various stages of processing.
In the past, mechanical sifters have been used to classify such particulate products. In such devices, perforated plates or metal screens are employed to trap particulates larger than the mesh size of the screens or plates. The retained large particles must be periodically removed. Such sifters are bulky, have numerous moving parts and are thus amenable to failure, and represent significant capital cost. Examples of commercial sifters include centrifugal sifters available from Prater Industries, Inc., Cicero, Ill., as the Roto-Sieve™, and the Roto-Trap™. Sifters generally also produce shearing of the particles, which is generally undesirable. The amount of “fines” often increases as a result.
It would be desirable to provide a classifying apparatus, or “classifier,” which is free of moving parts, yet which is capable of efficiently classifying a moving particle stream by removing large particles, agglomerates, foreign matter, etc., from the particle stream.